For many years, the standard way of obtaining heavier metals such as gold, silver, copper and iron from a gravel mixture containing such metals was to subject the gravel containing the metals to the scouring action of a sloped sluice box while washing the gravel with water to carry away the clay in the mixture. The use of such a method to obtain the finer particles in the mixture, however, has always had disadvantages. This is so because the quantity of water required to properly separate the clay from the metals is such that it would carry away the finer metallic particles without depositing them in the sluice. If the quantity of water is decreased, the gravel will not be properly conveyed down the sluice and it will simply deposit in the sluice, again without separating the fine metals.
A further technique used for separating the smaller particles has been to deposit the gravel on a mesh and shake the mesh. The smaller particles, of course, will drop through the mesh where they can be collected. Again, however, this technique is unsatisfactory to obtain fine materials because they are usually combined with clay and the clay cannot be separated from the metals without the use of water which, when used, causes the aforementioned difficulties.